Bass Driver Size - how much better is an extra inch?

Honestly, bass drivers should be 8” or bigger in my opinion.  10” or 12” is even better.  Thats why you should just augment the low end with a sub but many 2 way speakers today with 6” drivers produce sufficient bass.

If you sit there though and go up a line of speakers, the drivers get bigger and the sound fuller.  I’ve found that no 5” drivers can cope with the abuse I want to throw at them.  It’s got to be at least 6” if not 7”.  I learned this early on in the Paradigm line.  The difference between dynamics between studio 10’s and studio 20’s was pretty substantial.  The bigger driver on the 20’s vs the 60’s would even play louder.

So yes, with regards to bass drivers, bigger is better!

If you think you can't put everything in the same box you haven't been paying attention to newer designs. The Devaliet Phantom Premiere will go down to the teens with two 6.5 inch drivers. Genelec 8361A in room down in the 20's same with Dutch and Dutch. 

Cabinet design and driver excursion  more important than diameter.

I've never owned speakers with less than 10" bass drivers but I suspect a dual 8" driver arrangement would be fine for even larger rooms.  (3)8" drivers, probably better though.  Obviously, implementation is key.  When I went from dual 10" drivers to dual 12" drivers, the difference was subtle in my 16x28 listening room.  A little deeper, yes.  But the 10" were certainly plentiful.  I love being able to "feel" the music.  I have a hard time believing 6.5" "bass" drivers would give me anywhere near the same sensation.

The half inch, as a single metric, is irrelevant.

Much more important is the motor quality, room acoustics, and integration.

I know some 6 1/2" drivers I like a lot more than some 8"s. :0